1. Field of Invention
This invention relates to surge limited electronic ballasts for gas discharge lamps and, in particular, to a ballast that can rapidly provide low voltage for the electronic devices within the ballast despite the surge limiting circuitry.
2. Prior Art
A gas discharge lamp, such as a fluorescent lamp, is a non-linear load to a power line, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. Once the lamp conducts, the current will increase rapidly unless there is a ballast in series with the lamp to limit current.
A resistor can be used as a ballast but a resistor consumes power, thereby decreasing efficiency, measured in lumens per watt. A "magnetic" ballast is an inductor in series with the lamp and is more efficient than a resistor but is physically large and heavy. A large inductor is required because impedance is a function of frequency and power lines operate at low frequency (50-60 Hz.)
An electronic ballast typically includes a rectifier for changing the alternating current (AC) from a power line to direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 25-60 kHz. Since a frequency much higher than 50-60 Hz. is used, the inductors in an electronic ballast can be much smaller than the inductor in a magnetic ballast.
Converting from alternating current to direct current is usually done with a full wave or bridge rectifier. A filter capacitor on the output of the rectifier stores energy for powering the inverter. Some ballasts include a boost circuit between the rectifier and the filter capacitor for increasing the voltage to the lamp. The filter capacitor has a large capacitance, on the order of 15 .mu.f, and represents a low impedance to the AC line voltage. When an electronic ballast is first turned on, there is typically a large current surge through the rectifier to the filter capacitor, charging the capacitor. When the filter capacitor charges to approximately the nominal voltage of the AC power line, the current has decrease substantially.
Many electronic ballast use what is known as a "flyback" boost circuit in which the energy stored in an inductor is supplied to the filter capacitor as small pulses of current at high voltage, utilizing the .delta.i/.delta.t characteristic of an inductor. U.S. Pat. No. 3,265,930 (Powell) discloses such a ballast. An inductor between the bridge rectifier and the filter capacitor in a ballast does not limit surge current because the inductor is relatively small, on the order of two millihenries, and has a DC resistance of only a few ohms.
A modern electronic ballast typically includes an integrated circuit in the front end of the ballast to operate the boost circuit and provide power factor correction. "Power factor" is a figure of merit indicating whether or not a load in an AC circuit is equivalent to a pure resistance, i.e., indicating whether or not the voltage and current are sinusoidal and in phase. It is preferred that the load be the equivalent of a pure resistance (a power factor equal to one). Many semiconductor devices not only provide suitable AC to DC conversion but provide a "universal" front capable of being connected directly to any line voltage between 120 and 277 volts.
Particularly at higher line voltages, the surge current can be considerable and some means for limiting the in-rush surge current must be provided. Even at lower line voltages, current limiting circuitry is desirable to minimize stress on the active and passive electrical components within an electronic ballast. Having limited the available current when a ballast is first turned on has the effect of significantly increasing the starting time for the electronics within a ballast. (Lamp starting is a separate consideration.) Typically, the low voltage for powering the integrated circuits within an electronic ballast is derived from the normal operation of the boost circuit. When in-rush current is limited, the charging time for all capacitances is increased. Worse, the starting time may become voltage dependent, being longer for lower line voltages.
One must be cautious about adding circuitry to a ballast to solve each new problem as it arises. Not only are the cost and complexity of the ballast increased but the added components can cause an increase in the power dissipated by the ballast, thereby decreasing the efficiency of the ballast.
In view of the foregoing, it is therefore an object of the invention to provide a fast starting, surge limited, electronic ballast.
Another object of the invention is to provide an electronic ballast in which the starting time is consistent, reproducible, and independent of line voltage.
A further object of the invention is to provide a fast starting, surge limited, electronic ballast with as few additional components as possible.
Another object of the invention is to provide an efficient, fast starting, surge limited, electronic ballast.